It is known in the prior art to absorb oxygen from oxygen-containing gases using various chemically binding agents to extract available oxygen from gas streams, such as air. For instance, it is known to use barium oxide, sodium manganese oxide, strontium oxide, mercury, copper chloride, praseodymium or cerium oxides, chrome oxides, strontium-chromium oxides, alkali metal nitrate-nitrites, and alkali metal peroxides to reversibly absorb oxygen from an oxygen-containing fluid.
Exemplary of the nitrate-nitrite oxygen absorption system, is U.S. Pat. No. 4,132,766 in which oxygen is extracted from air using alkali metal nitrate and nitrite molten salt liquids. The patent addresses the problem of decomposition of the molten salt liquids to oxides or superoxides. The patent further alludes to the problem with the presence of water and carbon dioxide in the feed air to an absorptive separation of oxygen from air using such a nitrate-nitrite system, as well as other known chemical absorptive separations of oxygen.
U.S. Pat. No. 4,287,170 disclosed an improvement in nitrate-nitrite absorptive chemical separation of oxygen from air, wherein additional trace quantities of oxygen are removed from an initial separation effluent, using an oxygen scavenger, such as manganese oxide. Other metallic oxygen scavengers are mentioned such as copper, iron, nickel, cobalt, vanadium, tin, chromium, lead and bismuth oxides. These oxides are not mixed with the nitrate-nitrite bath, but are contained in the separately operated closed-circuit scavenger subcycle.
U.S. Pat. No. 4,340,578 discloses yet another improvement in oxygen separation from air streams using a chemical absorptive separatory agent, including nitrate-nitrite molten salt baths, wherein such bath contains an additional amount of peroxide and superoxide. It is noted in this patent that the nitrate-nitrite molten salt bath is susceptible to decomposition into the respective metal oxide, peroxides and super oxides, which will have a detrimental affect on salt concentrations, as well as corrosion of process equipment.
In U.S. Pat. No. 4,529,577 it is noted that oxide levels in a molten salt bath or oxygen absorptive separation from oxygen-containing gas, had previously been maintained in the 1 to 2% range in order to minimize corrosion. The presently discussed patent teaches that these oxides should be maintained below 1 mole %, based upon sodium peroxide, in order to avoid extensive corrosion problems in such an overall process.
It can be seen that alkali metal oxides including peroxides and superoxides used in chemical absorptive separations, such as the alkali metal nitrate-nitrite systems, have been known in the prior art to provide necessary catalysis to that separatory process, but are known in the prior art to be deactivated by water and carbon dioxide, which are typically found in air, the most prevalent source of an oxygen-containing gas from which oxygen would be separated. Additionally, it has been noted that nitrogen dioxide deactivates the alkali metal oxides present in an alkali metal nitrate-nitrite molten salt bath, despite the utility of nitrogen dioxide to avoid the decomposition of the nitrate-nitrite system. Accordingly, a problem exists in the prior art with the use of alkali metal oxides as catalysts for a nitrate-nitrite oxygen separatory system. Such alkali metal oxides are typically removed from a continuously operating process by reaction with the materials of construction of the process plant, reaction with the feed impurities presently existing in untreated air, such as water and carbon dioxide, and by vaporization of the alkali metal oxides at the high temperatures of operation necessary for alkali metal nitrate-nitrite molten salt bath separatory systems. Once the alkali metal oxide levels in the nitrate-nitrite salt mixture are removed or reduced in concentration, this oxygen separatory reaction does not occur at commercially feasible or economic rates.
In order to overcome this problem with alkali metal oxide catalysts in chemical absorptive separatory systems, such as the alkali metal nitrate-nitrite system, it has been suggested to replenish the alkali metal oxides continuously during the continuous operation of the underlying process. Alternatively, it has been suggested to generate additional alkali metal oxide species in situ, presumably by the decomposition of the nitrate-nitrite system to the detriment of that systems concentration in the overall process.
Another teaching of the necessity of alkali metal oxides for catalysis in such systems is disclosed (F. Pariccia and P. G. Zambonin, J. Phys. Chem. 78, 1693 [1974]). Such alkali metal oxides are present in the form of O.sup.2-, O.sub.2.sup.2- and O.sub.2 - (P. G. Zambonin, Electroanalytical Chem. and Interface Electrochemistry, 45, 451 [1973]). Additionally, it is also taught elsewhere in the prior art that water and carbon dioxide react with the alkali metal oxides in molten alkali nitrates, as set forth in (P. G. Zambonin, Anal. Chem. 44, 763 [1972]; P. G. Zambonin, Anal. Chem. 43, 1571[1971]). The art has recognized that the removal of these alkali metal oxides by any mechanism or theory, results in the slow kinetics of reaction for the oxygen uptake in a nitrate-nitrite system. as set forth in (F. Palimisano, L. Sabbatini and P. G. Zambonin, J. Chem. Soc. Faraday Trans. 1, 80, 1029 [1984]; D. A. Nissen and D. E. Meeker, Inorg. Chem. 22, 716 [1983]). However, the art has recognized that such alkali metal oxides can be generated in situ to replace those that are lost by various mechanisms, but this results in a decrease in the alkali metal nitrate-nitrite (C. M. Kramer, Z. A. Munin and K. H. Stern, High Temp. Sci. 16, 257 [1983]).
Transition metal oxides are known to have extensive redox chemistry (F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 4th Ed. John Wiley and Sons. Inc. New York, 1980), and in contrast to alkali metal oxides, form less stable oxy-anions (K. H. Stern, J. Chem Education. 46, 645 [1969]). The redox potentials of alkali metal nitrite is known from (M. H. Miles and A. N. Fletcher, J. Electrochem. Soc., 127, 1761 [1980]). The redox behavior of various transition metal compounds in molten salts has also been discussed in the prior art (D. H. Kerridge "Molten Salts as Nonaqueous Solvents", The Chemistry of Nonaqueous Solvents. J. J. Lagowski, ed., Academic Press, New York, 1978, pages 269-329). Finally, it is known in the prior art that oxide formation from a nitrate-nitrite molten salt bath can be suppressed by the introduction of additional amounts of nitrogen dioxide (E. Plumat. A. Labani and M. Ghodsi, J. Electrochem. Soc. 130, 2192 [1983]).
The present invention overcomes the problem of adequately catalyzing a chemical absorptive separation of oxygen from a oxygen-containing gas, wherein corrosion problems are minimized, water and carbon dioxide contamination do not constitute significant operational problems and an unexpected heightened activity is recognized to the benefit of the overall oxygen separatory process.